Historical Changes in Dairy Cattle

Over the last several decades, the dairy industry has seen substantial changes in cattle genetics, primarily focusing on traits that enhance milk yield and quality. The Council on Dairy Cattle Breeding provides extensive data that illustrate how dairy cows in the United States, especially Holstein, Jersey, and Brown Swiss breeds, have changed over time.

Historically, a cow would have a calf, milk for about 10 to 12 months, and then enter a non-lactating, or “dry,” period before beginning the cycle again. The average milk yield expected from a mature cow over a 10-month lactation cycle has nearly doubled since 1960. This increase in milk production is not only due to improved genetics but also advancements in nutrition, management, and housing.

Genetic and Phenotypic Changes

About half of the changes observed in milk yield can be attributed to genetic selection. While selection for higher milk yield has been ongoing for many years, it wasn’t until the mid-20th century that substantial genetic improvements became apparent. For instance, beef breeds like Angus, which are not selected for milk yield, produce significantly less milk than dairy breeds.

Additionally, cows’ physical characteristics have changed dramatically. A comparison between a Holstein from 1917 and a modern Holstein shows that today’s cows are taller and leaner, with significant improvements in udder conformation. The modern udder is more symmetrical, further from the ground, and less prone to injury, which has facilitated increased milk production while maintaining resistance to mastitis.

Negative Consequences of Selection

While the focus on higher yield has led to beneficial traits, it has also resulted in some unfavorable consequences, such as a decline in cow fertility. The genetic trend in daughter pregnancy rates, a key measure of cow fertility, has shown a severe erosion over the past five decades. Although there has been a slight recovery recently, fertility levels remain below historic norms.

Charles Darwin’s observation that trade-offs must be made during selection applies to dairy cattle breeding as well. Increasing milk yield often comes at the expense of other traits, such as fertility. This challenge of correlated responses has long been an issue for animal breeders and has driven the development of more sophisticated selection indexes.

Factors Driving Genetic Change

Four key factors determine the rate of genetic change in dairy cattle:

1. Accuracy of Genetic Merit Determination: Accurately determining a bull or cow’s genetic merit is crucial. For sires with many daughters, accuracy is very high, but it is lower for young bulls and cows.
2. Selection Intensity: Through artificial insemination, thousands of offspring can be produced from a single bull. This enables breeders to select only the best sires, increasing the selection intensity. Embryo transfer and in-vitro fertilization also help enhance selection intensity for females.
3. Genetic Variation: A trait must have genetic variation for selection to be effective. For example, while it would be desirable for cows to produce chocolate milk, the genetic potential for this trait does not exist in the current cattle population, so it cannot be selected for.
4. Generation Interval: The speed at which generations are turned over—the generation interval—determines the rate of genetic progress. The shorter the generation interval, the faster the genetic change.

Conclusion

The dairy industry has experienced significant genetic changes in the last 50 years, primarily driven by economic needs, technological advancements, and a deep understanding of genetics. The application of economic merit indexes has allowed for more targeted selection, ensuring traits that contribute to profitability are emphasized while addressing correlated responses to selection. The industry’s future will likely continue to see advancements as geneticists refine selection processes and leverage emerging technologies to further enhance dairy cattle genetics.